Op-Amp-Based VEHICLE THEFT DETECTOR

A simple, low-cost device can effectively alert homeowners or occupants if a parked vehicle is moved or tampered with.

- P. BALASUBRAMANIAN

This hardwired detector features a main circuit with a buzzer alarm installed inside the house, while a sensor is mounted on the vehicle being protected.

The sensor should be positioned on the vehicle in a location shielded from heat, rainwater, dust, and dirt. Its leads must be routed to an accessible point outside the vehicle for easy connection to the wires coming from the main circuit.

For consistent protection, the sensor leads must remain connected to the main circuit whenever the vehicle is parked. Any disconnection, cutting, or shorting of the wires triggers the alarm and activates the LED indicator. The detector is powered by an external 12V DC adaptor, with a 9V dry cell providing backup during power failures. Fig. 1 displays the author’s prototype of the detector.

Circuit and working

Fig. 2 shows the circuit diagram of the vehicle theft detector. The system is built around quad op-amp LM324 (IC1), quad 2-input NAND gate IC 7400 (IC2), and a few other components.

The quad op-amp IC LM324 (IC1) functions as a voltage comparator. The IC1A comparator detects higher voltage output from the sensor (in the dotted box) when the connecting wires (between TP1 and TP2) are short-circuited or connected while someone attempts to tamper with the vehicle.